1. Field of the Invention
The present invention relates to a network system for transmitting vehicle data, and more particularly to an economical and reliable network system for transmitting vehicle data for controlling the operation of electrically controlled devices such as lamps and actuators.
2. Cross Reference
A diagnostic apparatus for a vehicle network system which comprises a multidrop bus connected by a plurality of nodes and having two ends connected together by a connector, and a diagnosis unit connected to both ends of the multidrop bus for diagnosing in what part of the multidrop bus trouble occurs after replacing the connector by the diagnosis unit, is disclosed in U.S. patent application Ser. No. 07/580,948 filed Sep. 11, 1990, corresponding to Japanese Patent Application No. 1-672392 filed in Japan on Mar. 24, 1989, the entire contents of which are incorporated herein by reference.
3. Description of the Related Art
As the number and the type of electronically operated devices incorporated in a vehicle have been increasing year by year, the wire harnesses for making connection between these devices have come to be ever length and the number of wires by transmitting data from switches to corresponding vehicle loads by means of a communication network such as a multidrop bus, a ring, and a star.
The article "Development of Multiple Wiring Systems in Automobile" by Tohru Uramoto and Teruhisa Inomata, Multiplex Technology: Serial Networks SP-742 published by Society of Automotive Engineering Inc. (Feb. 1988) discloses a multiplex wiring system comprising a central control unit, local control units and a transmission line for connecting the control unit with the local units. Since, however, the transmission line comprises a single wiring system, communication between the control system and at least one of the local units is interrupted if trouble such as breakage occurs in some part of the line.
Further, U.S. Pat. No. 4,771,391 discloses an adaptive packet length traffic control in a local area network system in which information received by the first node from a ring is processed therein and transmitted as new information to the next node through the ring. This information transmission process continues until the information is received by the final node and processed therein. When trouble occurs in some part of the ring, information is not transmitted to the nodes downstream of the damaged portion.
In order to overcome this drawback, a dual network system can be considered. However, a dual system has the following drawbacks:
(a) An increase in the number of wires raises the cost of the system.
(b) When dual wires are arranged along the same path. It is possible that both wires may be broken, and thus it cannot be guaranteed that their provision will increase the reliability of the network system. If, on the other hand, both wires extend along different paths, the wire arrangement becomes complicated and the wires very long. Further, the number of connectors must be increased.
(c) In such a limited area as the space between the doors and the main body of an automobile, the wires cannot be arranged in different paths. Thus, if a door is accidentally opened while the automobile is in motion, both wires are likely to be broken.